[clang] [HLSL] Update indexed vector elements individually (PR #169144)

[Justin Bogner via cfe-commits]

https://github.com/bogner approved this pull request.

This all looks correct. Minor comments inline.

Note: Might've been better to combine this and #169090 into a single PR, but there are comments on both at this point so it seems awkward to pivot. As such, I'll just duplicate my comment on the wording over there.

https://github.com/llvm/llvm-project/pull/169144